Digital imaging devices such as digital cameras typically include some kind of automatic focus and exposure feature. At one extreme, some digital cameras perform coarse exposure calculations to keep the image within reasonable exposure range, deferring fine focus and exposure calculations until the user presses the shutter button to the intermediate (S1) position. The result is considerable delay (often greater than one second) after the shutter button is depressed to the image capture (S2) position before the final image is captured. Although such an approach minimizes battery power consumption, it introduces shutter lag. At the opposite extreme, other digital cameras calculate focus and exposure continuously, even when the user is not intending to capture an image imminently. This approach consumes more battery power but shortens shutter lag. Still other digital cameras perform a coarse focus update using low-resolution frames when a lighting change is detected. Such an approach still requires a fine focus adjustment at S1, again resulting in shutter lag.
It is thus apparent that there is a need in the art for an improved method and apparatus for continuous focus and exposure in a digital imaging device.